1. Field of the Invention
This invention relates to a method and apparatus for an absorption refrigeration cycle and machine having thermal performance comparable to the superior thermal performance of known triple effect absorption chillers compared to existing single effect, double effect, or dual loop absorption chillers/heat pumps/refrigeration cycle machines and having substantially less complexity than known triple effect absorption chillers.
2. Description of Prior Art
Absorption chillers arc heat driven refrigeration machines in which a secondary fluid, the absorbent, absorbs the primary fluid, gaseous refrigerant, that has been vaporized in an evaporator. In a typical single effect absorption refrigeration system, water is used as the refrigerant and lithium bromide as the absorbent. Other refrigerant/absorbent pairs (solutions) have been used, or have the potential for use, in absorption cycles.
In a single effect absorption chiller, refrigerant vapor is produced in an evaporator at a temperature somewhat below that of the heat load. The refrigerant vapor is exothermically absorbed by a concentrated absorbent solution entering the absorber. The heat of absorption is then transferred to a heat sink, such as cooling water, at the absorber. The now dilute absorbent solution is pumped to the generator, where it is concentrated again and returned to the absorber. External heat is supplied to the generator to supply the energy required to separate the refrigerant from the absorbent. The refrigerant is condensed at the condenser and is returned to the evaporator, while the concentrated absorbent is returned to the absorber. A heat exchanger between the absorber and generator is also part of the system, exchanging heat to the dilute absorbent from the concentrated absorbent solution. This process is carried out between two pressures, a lower pressure in the evaporator-absorber section and a higher pressure in the generator-condenser section. The operating temperature limits of the refrigerant/absorbent combinations are determined by the chemical and physical properties of the solution pair.
The coefficient of performance (COP) of a single effect absorption chiller is typically about 0.5 to 0.7. COPs above 1.0 in a single effect cycle are not possible because, for example, the heat required to generate one pound of refrigerant is not less than the heat taken up when this pound evaporates in the evaporator.
The COP of absorption refrigeration machines can be increased by using a double-effect evaporation principle and a double-effect generator. In such machines, two generators are used, one at high temperature and pressure heated by an external source of thermal energy and a second at lower pressure and temperature heated by condensation of the vapor from the first generator. Condensate from both generators is conveyed to the evaporator. In this manner, the external thermal energy is effectively utilized twice in the high and low temperature generators, thereby increasing the overall thermal efficiency compared to single effect absorption systems. The thermal efficiency of such double-effect cycles is typically in the range of about 1.0 to 1.2. Such multiple stage absorption/refrigeration systems are taught, for example, by U.S. Pat. No. 5,044,174 which teaches a refrigeration machine having an evaporator section, an absorber section, condenser section and a regenerator section respectively divided into two stages. U.S. Pat. No. 4,520,634 teaches a triple-stage absorption refrigeration system having three generators wherein gas refrigerant produced in a first generator is used as a heating source of a second generator, gas refrigerant produced in the second generator is used as a heating source of a third generator, a cooling medium flow system through which a cooling medium flows through an absorber after flowing through a condenser, and a dilute solution supply system wherein a dilute solution containing a relatively high amount of cooling medium is supplied directly to each generator without passing through other generators. U.S. Pat. No. 5,205,136 teaches a triple effect absorption refrigeration system having a double-condenser coupling at a parallel or series circuit for feeding the refrigerant-containing absorbent solution through the high, medium and low temperature generators utilized in the triple-effect system. The high temperature condenser receiving vaporous refrigerant from the high temperature generator is double coupled to both the medium temperature generator and the low temperature generator to enhance the internal recovery of heat within the system and thereby increase the thermal efficiency thereof See also U.S. Pat. No. 5,390,509 which teaches a triple effect absorption cycle apparatus comprising first, second and third generators each containing an aqueous absorption fluid and operating at successively higher temperatures, first, second and third condensers operating at successively higher temperatures, and operatively communicating with the generators, first heat exchange means cooperating between the third and second generators, and between the second condenser and the first generator, for directing energy therebetween, one, two or three absorbers and one, two or three fluid loops for directing aqueous absorption fluid between absorbers and second heat exchange means for exchanging energy between aqueous absorption fluid flows in said loops, and one, two or three evaporators operatively communicating with the absorbers; U.S. Pat. No. 5,335,515 which teaches a similar triple effect absorption cycle apparatus as the '509 patent; U.S. Pat. No. 4,551,991 which teaches a multi-effect absorption refrigerating system having an evaporator, a primary absorber, a plurality of generators, a condenser and a direct contact type heat exchanger which is provided with auxiliary evaporator-absorber stages in a plural set whereby the evaporator-absorber stages are divided into a plurality of groups and connected so that a solution is sent from the final absorber stage of each group into each generator, thereby permitting the solution in the first generator of a plurality of generators to work at a lower temperature level than normal; U.S. Pat. No. 3,831,397 and related U.S. Pat. No. 3,742,728 which teach a multi-stage absorption refrigeration system employing a highly concentrated solution of refrigerant to obtain an increased refrigeration effect relative to the quantity of input heat to the system as compared with a conventional system comprising a multi-stage regenerator-condenser system and at least a one-stage evaporator-absorber system having a pressure elevating device therebetween; U.S. Pat. No. 3,717,007 which teaches an air-cooled double-effect salt solution absorption refrigeration machine having high and low pressure generator stages which is operated with a lower pressure in the evaporator than in the absorber; U.S. Pat. No. 3,266,266 which teaches a double-effect absorption refrigeration machine in which refrigerant is released from the absorbent solution into generating stages; U.S. Pat. No. 4,542,628 which teaches a coupled dual loop absorption system having two separate complete loops, each of which operates at three temperatures and two pressures, whereby the low temperature loop absorber and condenser are thermally coupled to the high temperature loop evaporator and the high temperature loop condenser and absorber are thermally coupled to the low temperature generator; U.S. Pat. No. 5,467,614 which teaches a dual circuit absorption refrigeration system comprising a high-temperature single-effect refrigeration loop and a lower temperature double-effect refrigeration loop separate from one another and provided with a double-condenser coupling therebetween, the high temperature condenser of the single-effect refrigeration loop being double coupled to both of the generators and the double-effect refrigeration loop to improve internal heat recovery and employing a heat and mass transfer additive in the lower temperature double-effect refrigeration loop to improve the performance of the absorber and the double-effect refrigeration loop; and U.S. Pat. No. 4,732,008 which teaches a triple effect absorption method and apparatus in which two single effect absorption circuits are combined with heat exchange occurring between a condenser and absorber of a high temperature circuit and a generator of a low temperature circuit, the evaporators of both the high and low temperature circuits providing cooling to an external heat load.
The dual loop triple effect configuration of the apparatus of the '008 patent employs two separate generator/condenser/evaporator/absorber circuits utilizing two different absorbent-refrigeration fluids. The two different circuits are separate and the fluids are not allowed to mix. This separation of the two absorbent salt-refrigerant fluids adds costs and increases the complexity and size of a dual loop triple effect chiller.